1. Field of the Invention
The present invention relates to a dielectric ceramic composition and monolithic ceramic capacitor using the same.
2. Description of the Related Art
The conventional ceramic capacitor is usually produced by the following process.
First, a sheet of a dielectric material coated on its surface with an electrode material to serve as an inner electrode is prepared. A material essentially composed of BaTiO.sub.3 is used for the dielectric material. Then, the sheet of the dielectric material coated with this electrode material is laminated with heat-pressing to form a monolithic body followed by firing at 1250 to 1350.degree. C. in an environment to obtain a ceramic monolithic body having inner electrodes. A monolithic ceramic capacitor is obtained by glazing outer electrodes electrically connected to the inner electrodes.
Noble metals such as platinum, gold, palladium or silver have been conventionally used for the material of the inner electrode of this monolithic. ceramic capacitor. However, these electrode materials are expensive although having excellent characteristics, rendering the production cost high. Therefore, a monolithic capacitor using base metals such as Ni as the inner electrode is currently proposed to reduce the production cost, and its application in the market being steadily increasing.
In the trend to make electronic appliances compact, high performance and low price, strongly required is a monolithic capacitor of even lower price, improved in insulation durability, insulating property and reliability, and having a large capacitance. Although it is advantageous to use an inexpensive monolithic ceramic capacitor in which nickel is used for the inner electrode for reducing the price of the electronic appliances, there is the problem that the insulation resistance, insulation durability and reliability extremely deteriorate when the electronic appliances are used under a high electric field strength because conventional dielectric ceramic materials are designed on the premise that they are used under a low electric field strength. In other words, there has been no monolithic ceramic capacitor capable of use under a high electric field strength when using nickel for the inner electrode.
For example, while the dielectric materials disclosed in Japanese Examined Patent Publication No. 57-42588 and Japanese Unexamined Patent Publication No. 61-101459 display a large dielectric constant, the grain size of the dielectric ceramic is large, thereby exhibiting deficiencies such that the insulation durability of the monolithic ceramic capacitor becomes low when it is used under a high electric field strength or the mean life span under the high temperature load test is short.
In the dielectric material disclosed in Japanese Examined Patent Publication No. 61-14611, there was a deficiency that the dielectric constant, or the electrostatic capacitance, becomes extremely lowered when the capacitor is used under a high electric field strength, although its dielectric constant obtained under a low electric field strength is as high as 2000 to 2800. There is also the deficiency that the insulation resistance is low.